1. Field of the Invention
The present invention relates generally to image processing apparatuses for carrying out a variable scale magnification processing for contracting or enlarging an original image and, more particularly, to an image processing apparatus having data corrected in enlargement of the original image.
2. Description of the Related Art
A digital copier, for example, incorporates an image processing apparatus for adding a variable scale magnification processing in accordance with a copying magnification set via an operation panel to image data that is read by dividing an original for each pixel.
In general, as variable scale magnification processing, such a processing is carried out that pixel data (which are, strictly speaking, information indicating the shade or tone of the pixels, hereinafter referred to as "pixel") are skipped in contracting an original image and that the pixels are increased in enlarging the original image.
More specifically, in the variable scale magnification processing, with the direction of variable scale magnification determined as a main scanning direction for reading, if variable scale magnification is, e.g., "0.5", every other pixels arranged in the main scanning direction are skipped. If variable scale magnification is "3", the pixels are increased so that the same pixels are arranged three by three in the main scanning direction.
In the specification, the number of increased pixels indicates the total number of pixels of the same density arranged by the variable scale magnification processing. Therefore, when the same four pixels including three pixels added to the original pixel are arranged, for example, the number of increased pixels is "4".
It is known that if an image is enlarged by the above-described variable scale magnification processing, image definition becomes degraded as an enlargement magnification becomes increased.
That is to say, when pixels of different densities are adjacent to each other, discontinuity of darker and lighter shades is distinct if darker shade pixels and lighter shade pixels are increased with each other. This deteriorates the tone of an enlarged image. In particular, when image data undergoes an edge emphasis processing for making contours such as characters or letters clear, the pixels between darker shade and lighter shade are of extremely lighter shade.
Thus, the image data subjected to variable scale magnification processing is normally subject to a smoothing processing for shading off the darker shade and the lighter shade. The density of each pixel is converted into an added average density of plural pixels corresponding to smoothing filters of predetermined matrix sizes by the smoothing processing, thereby suppressing a degradation in the tone of the enlarged image. If the matrix size of the smoothing filters is determined to be uniform in such a smoothing processing, the smoothing of densities is not achieved in accordance with magnification of enlargement (variable scale magnification). As the degree of smoothing becomes excessive, the resolution of images becomes decreased.
Accordingly, as disclosed in Japanese Patent Laying-Open No. 63-209274, a conventional image processing apparatus is structured such that smoothing filters of different matrix sizes are properly used dependently on magnifications in order to achieve an optimal smoothing processing.
If magnification is not integral number such as "2", "3", "4". . . but decimal number such as "1.4" or "3.5", the number of increased pixels is not uniform in variable scale magnification processing.
That is, if the magnification is "3.5", an image is enlarged by 3.5 times in a false manner in the variable scale magnification processing in which the number of increased pixels is, e.g., "3" for odd pixels and "4" for even pixels.
Conventionally, however, an optimal smoothing filter is selected in accordance with a designated magnification; however, the matrix size of smoothing filters (hereinafter referred to as "filter size") is uniquely determined with respect to the magnification. More specifically, all pixels are subject to an uniform smoothing processing by a single selected smoothing filter.
Thus, when variable scale magnification is decimal, there are such disadvantages that the filter size in smoothing processing is excessively large with respect to a smaller number of increased pixels and that the filter size is excessively small with respect to a larger number of increased pixels. In other words, there is a problem that smoothing is not achieved dependently on the number of increased pixels, thereby degrading the tone or resolution of an enlarged image.